Poseidons are complex and expensive to maintain, require special hoses, upstream design (can fail closed), and, as I understand it, the cover cannot easily be removed *underwater* for cleaning. The problem with special hoses is not only the expense, it's that, should one in the field fail and you don't have a backup, you cannot share hoses amongst the team...think standardization! When everyone is diving the same (or at least equivalent and interchangeable) gear, a gear failure can usually be managed within the team and a dive saved.
The problem with upstream designs is that they are overly complex, thus more prone to failure, and can potentially freeze in the *closed* mode. Again, while I'm not familiar with the Xstream specifically, upstream regs in the past would also freeflow when they reached very low tank pressures since the little condom inside the second stage would no longer create a seal.
So, if you are interested in the DIR philosophy, sell them. Otherwise, if you can accept these faults and don't want to be DIR compliant, keep them.